Electrons Flashcards
Features of electrons
Have a finite range
Rapid dose fall off
Reduce dose to ntt, does not deposit dose at depth
High surface dose
Wide penumbra, increasing with depth
Range of energies, 6,9,12, 15, 18 and 20MeV
Why use electrons
Superficial treatment
Useful in treating skin, nose, ears, chest wall, eyelids, scalp, limbs
Can provide uniform dose from surface to 6-8.5cm depth
Where is dose prescribed
To the 90% iso line
The depth of 90% line is approximately 1/3 of energy
E.g. 9MeV 90% line is at 3cm depth
Why do we use best contact
To reduce amount of electrons travelling through air
Equal dose distribution
Avoid pdd changes
Avoid changes in SSD
What happens if energy of electrons are increased
Higher depth
More scatter and penumbra
Higher surface dose
If doctor wants dose to skin, what would you do
Use bolus as you would want the 90% dose line to be on skin
What is extended SSD and why do we use it
SSD higher than 100
Used when there is a larger field
Extend if something is in the way
Increasing SSD —-> higher % line gets narrower and lose depth
The lower % line get wider
When do we use fixed angle
If patient is immobilised
To miss OARs
What are the problems with junctioning
Max dose at intersection
Could lead to cold spot on skin surface if max dose is reduced
Why do we use bolus
Increase surface dose
Flatten out irregular surfaces
Reduce beam penetration in some parts of the treatment field
Advantages of electrons
Good cosmetic results
Sharp dose fall off below the surface
Less absorption in bone and cartilage compared to SXR
Disadvantages of electrons
Expensive LINAC required
Greater radiation protection required
Field size limitation
%DD less accurate under 4cm
Dose i homogeneity on curved surfaces
Eyes; shields can cause scatter
Dose effected by heterogeneities such as bone and air
Difficult to predict dose for oblique incidences
Effect of air and bone on electron fields
Air - increase scatter and increase depth on central axis
Bone - absorbs dose
